Mathematics – Logic
Scientific paper
Oct 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2767s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 10, pp.2767-2779
Mathematics
Logic
10
Scientific paper
Shallow groundwater samples from the Carnmenellis area of southwest England collected from wells, boreholes, springs, adits and stream baseflow represent recently recharged waters of low salinity. Their major ion concentrations closely reflect the lithology of their host rocks which comprise the Carnmenellis granite and Devonian metasediments (and metabasite) of greenschist grade. Groundwaters from the granite are predominantly Na-Cl or Na-Ca-Cl type whilst those from the metasediment are Na-Ca-Mg-Cl-HCO 3 waters with a larger range of compositions. The major ion compositions are predominantly a function of water-rock interaction processes and the larger range of metasediment-water compositions probably reflects the greater lithological variation. The pH of groundwater samples circulating through granite is typically lower than that of groundwater in the metasediments, the range being 4.3 to 7.2 (median 5.7) and 4.7 to 8.0 (median 6.6) respectively. Rare earth element (REE) concentrations of 0.45 m-filtered groundwater samples have been determined by inductively coupled plasma mass spectrometry (ICP-MS). A large range of concentrations exists and many are below detection limits but levels reach up to 229 g/1 (1.6 m) total REEs. The REEs are strongly controlled by pH with higher levels in groundwaters of pH < 6. Chondrite-normalised profiles are light rare earth element (LREE) enriched compared to heavy rare earths (HREEs), with a common slight negative Eu anomaly: both features reflecting the compositions of their host rocks. REE profiles therefore appear to be controlled by water-rock interactions involving predominantly REE-enriched minerals. In the granite, the REEs are probably derived mainly from the accessory minerals monazite and apatite as well as the high-REE framework minerals biotite and muscovite. The source of REEs in groundwater from the metasediments is more uncertain but probably includes clay minerals, chlorite, and carbonate. REE compositions normalised to shale have flatter profiles, although many are slightly convex with higher normalised Sm, Eu, and Gd values. A few metasediment-derived waters with high total REE concentrations have LREE-enriched shale-normalised patterns. It is suggested that the bulk of the REEs in these few samples is colloidal rather than totally dissolved. Groundwaters from the metasediments have a notable depletion in Ce which is distinct from waters in the granite. This depletion is problematic in that it is difficult to see how redox processes could be the dominant control, since groundwaters from the metasediments are unlikely to be more oxidising than those from the granite. Likewise, variations in pH fail to totally explain the distinctions in Ce concentration and La/Ce ratios between the two water types. It is therefore suggested that the Ce contents are largely source-related rather than process-related. The observed REE compositions of granite- and metasedimentderived groundwaters from the Carnmenellis area are so distinctive that the REEs in general and Ce in particular might be useful as future tracers in studies of water-rock interaction and groundwater provenance.
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